Functional colloids from a lignin-rich bioresidue and their application

Gerrow, Alex (2024). Functional colloids from a lignin-rich bioresidue and their application. University of Birmingham. Ph.D.

Preview

Gerrow2024PhD.pdf
Text - Accepted Version
Available under License All rights reserved.
Download (7MB) | Preview

Abstract

This thesis details the extraction and characterisation of a lignin-rich bioresidue from cocoa bean shell (CBS), the formation of particles using antisolvent precipitation, and applications in emulsions and microfluidics. A co-product of the food industry, there is an estimated 600 kt of CBS produced annually. Lignin, an aromatic component of CBS, is of interest to researchers aiming to develop analogues to current consumer goods from sustainable feedstocks, and as a natural source of aromatics has properties of interest such as hydrophobicity and anti-microbial effects. Lignin liberation from biomass typically involves depolymerisation by processes such as bleaching, often produced as a byproduct of commercial processes such as the Kraft paper pulping process termed Kraft lignin, however, this research uses mild acid assisted ethanol extraction. The bioresidue characteristics and co-products were characterised using analysis techniques such as FTIR and GCMS, identifying the presence of molecular surfactant, termed lipid. The extracted bioresidue which contained the lipid fraction (CBS-LC) was processed further to remove molecular surfactant, generating a second ‘lipid removed’ extract (CBS-LR). It was found that hydrodynamic flow-focusing microfluidics was unsuitable for a proposed desorption approach to antisolvent precipitation in water. An alternative continuous process for colloidal lignin-rich particle (CLP) production was generated by setting up a coaxial jet mixing device (CJM). By varying processing conditions to explore the effect of Reynolds number and volumetric flow rate ratio, non-aggregating particles were precipitated with average diameters between 150 and 600 nm. The aqueous dispersions were stable with respect to particle size for at least 14 days, for both types of extract.

The impact of both lipid and co-extractives from the crude bioresidue was assessed by use of CLPs as Pickering particles in emulsion studies. It was found that the emulsions containing crude CLPs were less efficient at stabilising a 1 in 10 sunflower oil-in-water emulsion than Kraft lignin analogues. This was theorised to be the result of the presence of co-extracted molecular surfactant, which may have had a competitive adsorption effect with the CLPs at the oil and water interface. The lipid fraction of the CBS extract was removed, and the resultant change in measured contact angle offered an indication that the CLP surface properties varied from hydrophobic to hydrophilic. It was also found that lignin-rich CLPs could be transferred from aqueous to non-aqueous dispersion in the presence of a molecular surfactant.

The application of the aqueous and non-aqueous CLP dispersions as a particle dispersion for ghost particle velocimetry (GPV) was explored, motivated by the fact that non aqueous tracer particle systems for GPV have not yet been reported. GPV is a technique for flow field imaging introduced in the last 10 years, allowing development of real-time quantification of flow fields in the focal plane and different cross-sections of a microfluidic channel using particles with a diameter of < 200 nm. This is currently achieved using a polystyrene based particle dispersion in the aqueous phase, and by demonstrating that lignin-rich CLPs can equally quantify flow fields, there potential to substitute these, which is an example of waste valorisation and environmentally sustainable substitution of a material. Application examples of this novel particle system were found not only to agree with theoretical flow velocity values but were able to quantify flow simultaneously in static and dynamic flow systems in both aqueous and non-aqueous phases. Whilst this thesis has demonstrated the ability to visualise and quantify flow fields using CLPs, the systems presented contain molecular surfactant in each instance.

This thesis contributes to the understanding of extraction and utilisation of lignin-rich bioresidue from CBS, highlighting its potential for use as a source of colloidal particles. Combining the spectroscopic techniques with characterisations such as contact angle offer insights into how particle behaviour is impacted by composition, paving the way for future advancements in creating dispersed particle systems with potential applicability in flow field imaging. Additionally, the first example of non-aggregating colloidal tracer particles in a non-aqueous system offers great potential for imaging not only the velocity of non-aqueous systems in confined channels, but formation of droplets; an area of significant interest in chemical micro-reactor systems.

To advance research in this area, it is recommended that initial work on the interactions between lignin and co-extracted surfactants are developed with respect to non-aqueous phase dispersions. Furthermore, there is an opportunity to explore the variation of the wetting characteristics of CLPs by employing novel solvent-antisolvent pairings. This would enable greater variation in the polarity of solvents in which CLPs can be dispersed. . In combining modified wettability with the adjustable particle diameter in a CJM, CLPs tailorable in both diameter and wettability may be produced, which could substitute a variety of current food-grade Pickering stabilisers.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):